The present invention relates in general to packages for semiconductive devices. More particularly, the invention relates to semiconductor packages that include stamped or etched metal lead frames. The invention is specifically adapted to, and will described in connection with, a 40-lead dual-in-line ceramic package, or Cer-Dip, but it is not so limited; the invention can, in fact, be applied to substantially any lead frame used in ceramic packages.
Ceramic packages have three essential parts. A ceramic substrate has a central cavity or depression that is metallized and plated to receive the semiconductive device or "chip". In Dip packages, the substrate is rectangular, and in "quad" packages, the substrate is square. The lead frame is secured to the upper surface of the substrate and includes a plurality of metal leads extending from the edge of the cavity to and beyond edges of the substrate. In Dip packages, the leads extend to the two long sides of the rectangular substrate and are there bent down to form two parallel lines of leads that are adapted for insertion into printed circuit boards. In quad packages, the leads extend over the edges on all four sides. Lastly, and after the chip has been bonded in the depression and wired to the leads, a ceramic cover is secured over the top to hermetically seal the chip.
During manufacture, the outer ends of the leads are all integral with a surrounding frame which holds them in place (hence the name "lead frame") and there may also be intermediate tie-bars between the leads assisting in the same function. At some point after the lead frame is secured to the substrate, the frame and tie bars are cut away.
Lead frames are made by either stamping or etching. Etching is a relatively slow process, but it is advantageous in that required photomasks are not expensive. Etching is thus preferred for low-volume production, particularly where complicated patterns are involved. Stamping is very rapid, but requires a very substantial investment in tooling, because as many as 15 progressive steps may be involved. Stamping is thus preferred for high volume production.
Lead frames are manufactured out of so-called glass-sealing alloys, which are alloys specifically designed to have a coefficient of thermal expansion about the same as the glass they will ultimately be embedded in. Kovar (trademark) is one such alloy and contains iron, cobalt and nickel. Alloy 42 is similar. Kovar is preferred for etched frames and Alloy 42 is preferred for stamping. These alloys are difficult to bond wires directly to. Accordingly, the alloy may be provided with a stripe of vapor-deposited aluminum which covers the lead tips, or a dot of gold may be applied over the lead tips by selective plating.
The progress of microelectronics has been miniaturization, putting more and more electronic functions on a single chip. Whereas a few years ago, a 14-lead Dip was practically standard, today 40 and even 64-lead packages are commonplace. The chip itself and the cavity in the substrate have remained about the same size (roughly 1/4 in. square), however, so the leads have had to become progressively finer and be more closely spaced. In a 40 lead package, for example, the leads (at the tip) may be 0.012 in. wide and spaced 0.012 in. apart. Because no tie bars can be placed on the portions of the leads to be embedded in the glass, the very fine leads can move about, particularly during the embedding operation, and one short circuit -- e.g. two leads touching, will ruin the entire package.
Accurate lead positioning is even more important -- in fact it is absolutely essential -- when automatic equipment is used to insert and wire bond the chip.
One solution to this problem is to extend the lead tips over the edge of the cavity, and have them connected by an internal tie-bar. A multi-layer ceramic package is then used; the lead frame is embedded on the surface of a substrate having a hole instead of a cavity. A punch and die then trim off the internal tie-bar. A second substrate, including the gold-die-attach pad forming the bottom of the cavity, is then secured to the upper part.
While prior art relevant to the present invention, other than frames with internal tie-bars as noted above, is not known, certain patents are of interest. Thompson, U.S. Pat. No. 3,802,069 discloses a frame having a central pad held by two leads during manufacture to prevent pad misalignment, one lead being severed prior to encapsulation. Lincoln, U.S. Pat. No. 3,893,158 discloses a complex frame including both upwardly and downwardly offset portions, produced by cutting a frame in half and rotating one-half 180.degree.. Frames for plastic Dip packages are disclosed in U.S. Pat. No. 3,629,668 of Hingorany, and include a bonding pad and supporting tie bar attached to, but of a material distinct from, the remainder of the frame.